An investigation by the environmental authorities found that the smoke was caused by farmers burning straw in Hebei Province.

This time every year, after harvesting wheat, farmers in areas around Beijing set fire to straw to pave the way for the next crop. For Beijingers, however, this is an annual reminder that the city's pollution is not only caused by factories and automobiles within the city and sandstorms from the north.

Beijing is far from alone in suffering from smoke emanating from wheat fields. This is a widespread problem across northern China and parts of the south.

Officially, burning straw is banned, due to pollution and the increased risk of fire. But the ban has never really worked.

In many cases, a more sophisticated approach and smoother co-operation between different government departments are needed to solve a seemingly simple issue.

Experts have floated many inspiring ideas about how to deal with the straw, ranging from turning it into forage or fuel to pulverizing it with modified harvesting machines before using it as natural fertilizer. Some of these solutions can even help boost farmers' incomes.

But all these solutions require additional investment.

Considering farmers' generally low incomes, it would not be well advised to impose additional financial pressures on them by making the adoption of such technology or devices compulsory.

In terms of solutions that could benefit farmers economically for example, the introduction of a device that could transform straw into forage it is also important to make sure that the benefits can be sustained.

The harvest season is a hectic period for farmers, who have to reap tons of crops and immediately plant new ones for the next season. It will not be sensible, either, if the solution requires farmers to spend a lot of time in doing away with the straws.

Indeed, it could be a rather complicated issue, requiring creative thinking based on local conditions and co-operation between agricultural, environmental departments and local governments.

Before the real effective solution is worked out, Beijingers and people in other cities may still have to tolerate a few days of smoke from the fields.